Two come to mind:
10.6: No new consumer facing features, just a big ironing out of 10.5.
10.7: Removed 2D Spaces layout to a left/right one. Totally ruined my workflow. Also got rid of scroll-bars.
10.8: ?
10.9: Not exactly OS level, but iWork took a huge backslide feature wise.
10.10: Blurs everywhere. It can put a hit on overall performance of applications, especially visual ones like Photoshop+RAW images or video editing.
Nothing comes to mind for [10.8, 10.9] which might say a bit to your point. However, Messages has been in since 10.8 and items are still out of order? They need to iron out just like they did with 10.6, otherwise the bugs and deep issues will compound year after year.